1. Field of the Invention
This invention relates to rock drilling and more particularly it concerns the formation of large diameter holes in rock outcroppings on a sea bed.
2. Description of the Prior Art
The drilling of large diameter holes in the sea floor is known. The prior art includes the provision of a jack-up platform having legs which extend down to the sea floor and support the platform above the sea surface. A large diameter tubular drill guide is attached at its upper end to the platform and extends down to the sea floor. A drill or cutter assembly and a drill string or drill pipe is inserted into the guide at the platform and is lowered down through the guide to the sea floor. A drive mechanism on the platform turns the upper end of the drill string or drill pipe to rotate the cutter assembly which bores into the earth or rock to form a hole. Stabilizer assemblies are arranged along the drill string or drill pipe and these serve to center the drill string as it rotates and moves down inside the guide. This prior art is shown in a publication entitled Wirth Airlift Drilling Rigs, Pile Boring Equipment and published by Maschinen Und Bohrgerate-Fabrik, Alfred Wirth & Co. K.G. 514 Erkelenz/Fed. Republic of Germany P.0. Box 1327/1329. Other publications of interest in the prior art are Review of Reverse Circulation Air Lift Methods for Big Hole Drilling by J. H. Allen, SME Transactions, June 1977 pp. 86-93, Drilling Large Diameter Holes by J. H. Allen from Australasian Oil & Gas Review and reprinted by Smith Tool Co. division of Smith International Inc., Compton California and Floating Rigs Build Piers by H. K. Glidden, published in Roads and Streets, February, 1965.
A problem arises when the above described arrangement is used for drilling large diameter holes in areas where the sea is especially turbulent. This is because the large waves and currents produced by turbulent seas impose a great load on the large diameter drill guide and may cause it to bend or fracture or break away from its attachments to the drilling mechanism.
Another problem that arises when drilling large diameter holes in rock outcroppings in the sea bed is due to the fact that the sea bed in such locations is usually rough and sloping. As a result, the drill is likely to be thrown laterally off position when it initially encounters the rock at the sea bed. In addition, the sudden shock produced by engagement of only part of the drill with any projection of the rocky sea bottom could cause stresses that would be harmful to the drill or the drilling mechanism.
The prior art shows various arrangements for positioning drill bits at a sea bed. U.S. Pat. Nos. 3,114,420, 3,252,529, 3,486 555 and 3,672,447 all show means including cables or relatively flexible pipes for guiding a drill down to a drilling location. These arrangements, however, are all associated with a floating vessel. Moreover, they would not be suitable for drilling into a rock outcropping because they rely on forming a stabilized base in a relatively soft overburden on the sea bed. In addition, the holes proposed to be drilled with these prior arrangements are of small diameter and it is not important that the drill and drill string be maintained in precise alignment between the drilling mechanism and the hole being drilled. U.S. Pat. No. 3,248,886 shows a riser pipe for a floating rig but this riser pipe is not maintained in alignment; in fact, a flexible joint is provided along its length.
U.S. Pat. Nos. 2,354,142, 946,841, 427,149 and 3,672,177 all propose structures having moveable elements along their lower edge to conform to an uneven sea bottom but none of these patents relate to drilling operations and none of them provide any indication of how to avoid the problem of starting a drill bit smoothly and accurately in an uneven rock surface without damage to the drill bit.
The present invention, in one aspect, solves the above-described problem of maintaining a drill and drill string in proper alignment for drilling large diameter holes in a rocky sea bottom where the sea is turbulent. According to this aspect of the invention there is provided a drilling rig with legs which rest on the sea floor and which support a platform up above the surface of the sea. A rigid open framework drill guide extends down from the platform to the sea bottom to guide the drill and drill string. The drill guide may comprise a plurality of vertically spaced apart short cylindrical sections held in place by vertical columns. Centering elements are provided to be guided vertically along rails in the drill guide; and these elements hold the drill string centered and in proper alignment with the drill. Cables may be provided at the lower end of the drill guide and these may run through sheaves in the adjacent supporting legs and up to the platform to winches which serve to maintain proper positioning of the lower end of the drill guide. Because of the open framework construction of the drill guide it provides minimal surface to the turbulent sea and accordingly waves and currents do not impose excessive stresses on the drill guide or its supports.
In another aspect of the invention the lower end of the drill guide is provided with a flexible container and vertically moveable reinforcing rods are distributed about the lower periphery of the drill guide outside the container. When the drill guide contacts the uneven sea bottom, the reinforcing rods extend by different amounts down to the uneven rock surface according to the elevation of the rock surface at their particular locations. A hardenable substance, such as concrete, is then introduced into the lower end of the drill guide to fill the flexible container and to fully or partially fill the lower end of the drill guide. The reinforcing rods serve to keep the concrete from flowing laterally under the drill guide and bursting the container. The concrete will conform itself at the bottom to the irregularities of the rock surface but it will be substantially flat at its upper surface. After the concrete hardens, a rock drill is lowered down through the drill guide to the hardened concrete. Drilling begins at the flat upper surface of the concrete and continues down through the concrete and into the underlying rock. With this arrangement the drill is not subjected to lateral skipping or undue stresses due to localized contact with the sea bed. Also the concrete provides a gradual transition for the drill into the hard rock material of the sea bed.
The invention may be carried out in various ways and is not limited to the specific way described below which is the best mode contemplated by the inventor.